1. Field of the Invention
The present invention relates to a light beam recording device and to a disk used in the light beam recording device.
2. Description of the Related Art
A laser computer output microfilmer (laser COM) is known as a device for recording characters, images or the like onto a recording material by use of a light beam (Japanese Patent Application Laid-Open No. 55-67722). In this device, a laser beam is scanned based on, for example, computer output information, so that information, such as characters, are directly recorded onto a recording material such as a microfilm. In this type of light beam recording device, the light beam is deflected by a deflecting means such as a polygon mirror or a galvanic mirror and is then imaged onto a recording surface of the recording material by a scanning lens so that information is recorded onto the recording material.
The above-described device uses an f.multidot..theta. lens having an imaging relation expressed by the following formula (1). The recording material is disposed at the focal point surface (hereinafter, "image surface") of the f.multidot..theta. lens. EQU s=f.multidot.tan.theta. (1)
wherein
s: distance from the optical axis to the imaging point PA1 f: focal length of the f.multidot..theta. lens PA1 .theta.: angle of incidence of the light beam with respect to the optical axis
It is possible that the image surface of the light beam deflected by the deflecting means and transmitted through the f.multidot..theta. lens is not planar due to the crystal inside the f.multidot..theta. lens being irregular or due to other reasons. For example, as illustrated in FIG. 18, a laser beam is irradiated from a semiconductor laser 100 and is then transformed into parallel light by a collimator lens 102. After being deflected along the scanning direction (the direction of arrow C in FIG. 18) by a polygon mirror 104, the laser beam is imaged by an f.multidot..theta. lens 106 onto a recording material wound on a drum 108. However, the focal position (specifically, the beam waist position) of the laser beam deviates from the recording surface due to the deflection angle, resulting in the curving of the image surface as shown by the broken line 110 in FIG. 18 (hereinafter, "curvature of field surface 110").
In FIG. 18, at portions at which the beam waist position of the laser beam deviates from the recording surface (especially in vicinities of the end portions of the curvature of field surface 110), the beam diameter of the light beam illuminated onto the recording surface is relatively large compared to the beam diameter at the beam waist position, and the energy of the light beam irradiated onto the recording surface per unit area is low. Therefore, a drawback arises in that portions of the character, image or the like recorded on the recording surface are unclear. In recent years, more demands have been made of light beam recording devices which can be applied to output devices or the like for printing and which can record even larger sized images. As the size of the recorded image increases, the image quality deteriorates due to the above-mentioned phenomenon.
In order to solve this drawback, a method has been considered in which the curvature of field surface is corrected so as to become planar by an electrooptic lens which uses a PLZT electrooptic ceramic or the like and which allows arbitrary, electric control of the beam waist position of the light beam. However, because light transmittance of the PLZT electrooptic lens is low and scattering is high, the power of the light beam cannot be utilized effectively. The PLZT electrooptic lens is therefore unsuitable for recording of large-sized images. Further, as electrooptic lenses are expensive, a drawback arises in that the cost of the light beam recording device increases.
Further, Japanese Patent Application Laid-Open No. 3-17610 discloses a scanning optical device in which a wedge-shaped prism is disposed before a deflecting means which is disposed on an optical path of a light beam. The prism is moved synchronously with the deflection of the deflecting means. When the wedge-shaped prism varies a position above the prism through which the light beam is transmitted, the optical path of the light beam can be varied, and consequently, the beam waist position of the light beam can be varied. Accordingly, if the prism is moved so that the curvature of field is corrected and the image surface becomes planar, the beam waist position of the light beam can be made to coincide with the recording surface.
However, there are few cases in which the curvature of field surface is a simple curved surface which is curved at a constant curvature of field (see FIG. 18). In order for the scanning optical device to make the image surface planar, it is necessary to reciprocally move the prism a plurality of times and in a complicated manner for each one scan of the light beam. When the light beam is scanned rapidly, the inertial force or the like of the prism is effected, and the prism cannot be moved accurately. Therefore, it is difficult to record the image or the like at high speed. Further, even if the movement of the prism can follow the scanning speed of the light beam, the vertical angle of the light beam varies along with the movement of the wedge-shaped prism. Therefore, a drawback arises in that the intervals between pixels vary, resulting in distortion of the image.